It constitutes, in particular, an improvement both to the French patent published under No 2543768, 2543768 corresponds to U.S. Ser. No. 595,739 filed Apr. 2, 1984 now U.S. Pat. No. 4,622,662 and to the French patent published under No 2519148, 2519148 corresponds to U.S. Ser. No. 452,481 filed Dec. 23, 1982 now U.S. Pat. No. 4,583,820.
From the French patent published under No 2479981, and more especially from its second addition published under No 2496260, there is known an integral assembly permitting combination, within a long-distance transmision fiber, of a plurality of luminous fluxes having differing wavelengths, guided by an equal number of input fibers, or indeed, operating in the inverse mode, separation and distribution to a plurality of exit fibers of a plurality of luminous fluxes of differing wavelength guided together by a single transmission fiber.
In such a device, in which the entrance and exit fibers are disposed in the immediate vicinity of the focus of a concave mirror, the latter in a demultiplexer operational mode for example, converts the beam of mixed luminous fluxes received from the entrance fiber into a parallel beam directed towards a plane diffraction grating, and on the other hand focuses on the ends of the exit fibers the various monochromatic parallel beams diffracted by the grating.
The French patent published under No 2519148 describes a similar device, in which separation of the wavelengths is provided by selective multidielectric filter mirrors.
Such devices, whether they include gratings or dielectric mirrors, give better results with a parabolic concave mirror, rather than a spherical one. In fact, the spherical aberrations of a spherical mirror do not permit the achievement of the same degree of stigmatism quality as with a parabolic mirror.
The French patent published under No 2543768 indicates a firt improvement, applied to a grating device, by the formation of a plane dioptric element separating two optical media having differing indices between the concave mirror and the grating, thus making it possible to use a simple spherical mirror. The spherical aberrations of the mirror may in fact then be corrected by an apropriate selection of the indices of the two media of the dioptric element.
Unfortunately, it becomes evident that, in practice, the correction of the spherical aberrations by a plane dioptric element requires a relatively large difference in the indices n and n', for example such as n.gtoreq.2n'.sqroot.3. With currently available glasses, the result of this is differences in chromatic dispersion which are likewise large, and which involve chromatic aberrations and may compel inclination of the plane of flush alignment of the optical reception or emission fibers for the various wavelengths, which increases the difficulties involved in industrial production.
That which has just been stated concerning the problems involved in the use of a plane dioptric element to correct the spherical aberrations in a component making use of a spherical concave mirror and a separation of wavelengths formed by a diffraction grating would also apply in the same way to a component making use of a concave mirror and a separation of wavelengths formed by at least one selective multidielectric filter mirror such as is described in the French patent published under No. 2,519,148.